The invention relates to phased array antennas employing active RF modules containing transmit and/or receive amplifiers, and more particularly to a technique for achieving low sidelobes in such an antenna.
Phased array antennas which employ feed networks and comprising active transmit/receive microwave modules have been implemented and described in the literature.
Techniques for controlling the sidelobes of such systems also exist. One technique which has been used in the past to achieve low transmit sidelobes (tapered aperture illumination) is to use modules with different power outputs. This provides a stepped aperture distribution which produces low sidelobes adjacent the beam. Disadvantages of this technique are:
1. The steps in the aperture distribution lead to high sidelobes in the region away from the beam.
2. The requirement of modules having different power outputs leads to higher production cost.
3. The different output powers of the modules are obtained by varying the number of solid state devices in the output stage. This requires different combiners with different losses and phase error, thus making the system more complex.
4. Different driver chains are required leading to phase and amplitude tracking (between modules) over the frequency band thus tending to increase the sidelobes.
To get a tapered amplitude, varying the modules, supply voltages will change output power; however, the dc-to-rf efficiency decreases and phase tracking is difficult, particularly in the class C amplifiers often used. The use of class A amplifiers will produce varying output by simply varying the input; however, the efficiency will be poor since typically a 10 dB output power variation is required.
Another technique which requires only identical modules is to decompose the transmit aperture into equal power segments which necessarily contain different numbers of radiating elements for a tapered illumination. This requires phase shifters downstream from the transmit amplifiers introducing one-way losses of 1 dB or more.
One purpose of the invention is to provide an electronically scanned phased array antenna for radiating low sidelobe beams using identical solid state modules without the aforementioned disadvantages.
Another purpose of the invention is to provide a phased array antenna which employs identical modules to achieve radiation patterns having low sidelobe levels, and avoids the need for lossy phase shifters between the transmit amplifiers and radiating elements.